The present invention relates to the detection of fatigue cracks or defects in materials and, more particularly, to an apparatus and method for detecting fatigue cracks in gas turbine engine components and airframe components, such as fuselages, wing assemblies, tail sections and the like, by using infrared thermography.
Fatigue cracks occur in a wide variety of both metallic and nonmetallic aircraft parts. Low cycle fatigue (LCF) cracks represent one of the most prevalent failure mechanisms for material that has any cyclic stresses applied to it. This is particularly true for any of the rotating parts in a gas turbine engine. At the same time LCF cracks are a difficult type of material defect to detect. Many methods of detection are employed within the aircraft industry in an attempt to consistently locate LCF cracks early in their growth to preclude a material failure.
Currently used nondestructive evaluation (NDE) methods for inspecting aircraft parts and gas turbine engine components for fatigue cracks or other defects, which could cause a failure of the engine or airframe, include surface wave ultrasonic testing, eddy current testing, acoustic emission evaluation and fluorescent penetrant inspection. Each of these methods is not without certain limitations and disadvantages; some methods require that the evaluation instrumentation contact the surface. Unique geometries of some components may restrict the evaluation techniques that can be utilized, and some of these methods are susceptible to errors and false indications from contaminants on the surface of the component being inspected or contaminants within the cracks or defects. Some NDE methods are susceptible to errors or false indications of the presence of a defect from surface roughness and other surface anomalies which do not result in a failure.
Generally, none of the above-described NDE methods is considered to be singularly capable of detecting LCF cracks with sufficient reliability and ease of application to have emerged as a preferred method. In the most critical of applications, two or more of these conventional methods are typically used in tandem as a crosscheck on one another.
Infrared methods of NDE overcome many of the aforementioned disadvantages associated with the NDE methods listed above. Infrared methods can be performed relatively quickly and easily and are also suitable for adaptation to automation without many of the limitations associated with the other NDE methods described above.
Infrared NDE methods operate on the premise that all matter continuously absorbs and emits electromagnetic radiation. The continual motion of the charged particles within a material results in the emission of electromagnetic radiation. The motion of the charged particles will increase with an increase in temperature and therefore the continuous emission of radiation from the material will also increase with an increase in temperature. The Stefan-Boltzman law states that the total energy radiated by a perfect black body is proportional to the fourth power of the absolute temperature. The ratio of the total emissive power of any body to that of a perfect black body at the same temperature is known as the emissivity of the body and is numerically equal to the absorptivity of the body. Cracks and defects may be detected because they will typically absorb more radiation and therefore have a higher emissivity and radiance relative to the relatively flat and smoother surface areas surrounding the defect.
A thermal imaging device for nondestructive testing using laser illumination and an infrared detector is disclosed in U.S. Pat. No. 3,808,439 issued to Renius. Renius teaches continuously scanning the entire surface of a specimen under test with a CO.sub.2 laser beam so that the amount of total incident radiation absorbed by the specimen is equal for the entire surface. The laser scanning causes an increase in the surface temperature with the heat propagating through the specimen. The differences in the heat transferred through different portions of the specimen are detected by an infrared detector and are then used to determine subsurface voids or defects.
Another NDE device and method using infrared radiation is disclosed in U.S. Pat. No. 3,499,153 issued to Stanfill. This invention detects flaws or inhomogeneities in the surface of a material under test by irradiating the material surface with infrared radiation to maintain the material at a substantially uniform temperature. Radiation reflected by the material is then detected by a radiometer.
A further method for detecting flaws in the surface of a tested material by detecting thermal emission from the material is disclosed in U.S. Pat. No. 4,232,554; this invention loads or places the part being tested under a uniform tensile stress normal to the crack and then detects thermal emission signals indicative of plastic deformation. The presence of a crack or deformity can then be determined from the thermal emission signals.
Other NDE devices and methods are disclosed in U.S. Pat. Nos. 3,462,602; 3,451,254; 3,206,603; 3,222,917; 3,434,332; 3,433,052; 3,427,861 and 3,401,551.
None of the above-referenced patents disclose the benefits derived by selective, localized electromagnetic radiative heating to improve the contrast between any crack, or defect, and the material surrounding the crack to improve the detection ability of very minute cracks, as small as about 0.01 inches in length. Additionally, there is no teaching in any of the above-referenced patents of the benefits derived from analyzing a transient response corresponding to a detected infrared image of the radiance from a selectively heated workpiece surface, to distinguish between defects which could result in a failure and other minor surface anomalies which are not of great concern.